Approaches are being explored to increase the therapeutic efficacy of cytosine arabinoside (ara-C) by the modulation of its intracellular metabolism. Recent work in this laboratory has demonstrated a highly significant relationship between the incorporation of ara-C into cellular DNA with the loss of clonogenic survival. This relationship is being explored further by defining the effect of specific nuleotide sequences on the formation of (ara-C) DNA and whether ara-C is incorporated into specific nuclear DNA segments. These biochemical parameters are being examined as correlates of biologic effect that lead to a loss of clonogenic potential, declines in progenitor self renewal capacity or the induction of terminal differentiation. The significant relationship between ara-C incorporation into DNA and cytotoxicity also defines a biochemical parameter for directly monitoring the modulating effects of other drugs on the metabolism of ara-C. Agents including thymidine, PALA, methotrexate, 3-deazauridine and pyrazafurin are being employed in studies with the human HL-60 AML line as a means of enhancing ara-C incorporation into DNA through the metabolic depletion of dCTP pools. The therapeutic efficacy of these approaches with ara-C will be investigated in a rat AML model by monitoring the formation of (ara-C) DNA in myeloblasts and normal cells as correlates of antitumor effect and toxicity. A similar approach is being applied in an ongoing phase I clinical trial which is designed to monitor the enhancement of ara-C chemotherapy with thymidine. This study will evaluate the effects of thymidine on depleting dCTP pools and increasing ara-C incorporation into tumor cell DNA. We will attempt to correlate these molecular parameters with the clinical response and with the clonogenic and self renewal capacity of leukemic progenitors. The overall objective of these studies is to develop more effective therapeutic approaches by enhancing the incorporation of ara-C into DNA. These studies are important in terms of providing a rational biochemical basis for the guidance of future clinical trials with ara-C.